An Inflatable Sail to the Oort Cloud

byPaul GilsteronNovember 12, 2008

Want to get to the outer Solar System quickly? Try this on for size: Two and a half years to reach the heliopause, six and a half years to get to the Sun’s inner gravitational focus (550 AU), with arrival at the inner Oort Cloud in no more than thirty years. A spacecraft meeting those targets is moving at 403 kilometers per second, roughly twenty times as fast as anything we’ve put into space before. Such a mission could perform useful astrophysical observations enroute, explore gravitational focusing techniques, and image Oort Cloud objects while exploring particles and fields in that region that are of galactic rather than solar origin.

The combined Oort Cloud explorer/gravity focus probe grows out of work by Gregory Matloff and Roman Kezerashvili (CUNY), Italian physicist Claudio Maccone and Les Johnson (NASA MSFC). Matloff, of course, has been studying solar sail technologies for decades, looking at missions that could reach velocities in the range of 0.003c-0.004c, with metallic sails that, parachute-like, pull a payload attached to diamond-strength cables. The cables (and the sail itself) can be wound around the payload enroute to provide cosmic-ray shielding and, in the case of true interstellar missions, redeployed upon arrival at a destination star.

That’s a familiar sail concept, but it’s one with a problem: Such designs do not scale well. In fact, as you increase the size of the spacecraft, the proportion of its mass that is devoted to cable rises rapidly with payload. Because of his long-standing interest in ‘generation’ ships, capable of carrying human colonies on millennia-long journeys to the stars, Matloff has a natural interest in refining the sail so it can be used in ever more ambitious missions. It’s natural to turn to the idea of inflatable beryllium sails, hollow-body sails that feature sail surfaces just tens of nanometers in thickness, with the body inflated by a low-pressure gas like hydrogen. Unlike the ‘parachute’ concept, the payload would be mounted on the space-facing surface — the inflatable sail is a ‘pusher’ model.

The concept goes back to Joerg Strobl, who first published it in a 1989 paper for the Journal of the British Interplanetary Society. And it’s a design that seems to scale well if properly deployed. The team studied two configurations, one a generation ship with inflated sail radius of 541.5 kilometers, a payload of 107 kg, and a separation between the sail faces of one kilometer. A second is a near-term extrasolar probe with sail radius of 937 meters, a 30 kg payload and a 1.8 meter separation. The numbers show how well the concept adjusts to different missions:

From the point of view of kinematics, mechanical stress, and thermal effects, the hollow-body solar photon sail scales well. Both conﬁgurations had a spacecraft areal mass density of 6.52 × 10−5 kg/m2, a peak internal gas pressure of 1.98 × 10−4 Pa, and a peak perihelion temperature of 1412 K. If fully inﬂated at the 0.05 AU perihelion of an initially parabolic solar orbit, both had a peak radiation-pressure acceleration of 36.4 m/s2 and exited the solar system at 0.00264c after an acceleration duration less than one day.

The new paper looks hard at the issues these designs face, including problems with the proposed 0.05 AU close pass by the Sun and the effects of solar radiation on sail materials and the hydrogen fill gas. The result is a modification of the near-term concept discussed above, with perihelion adjusted to 0.1 AU. The greater distance lowers the sail temperature considerably and reduces the need to replace hydrogen fill gas that will have diffused through the sail walls at higher temperatures. Even so, the team calculates that the gas must be replenished some fifty times during this solar acceleration process. The challenge is manageable:

To err on the side of caution, it is assumed here that a hydrogen reserve of 100 times the required ﬁll gas mass is carried aboard the spacecraft. This amounts to only 170 grams of hydrogen. If hydrogen ﬁll gas is dissociated from water as required, no more than about one kilogram of water is required. Even water-storage and dissociation equipment will not add more that a few kilograms to the payload and have a very small effect on spacecraft performance.

Also manageable is the constant ionization of beryllium sail atoms during the acceleration period, the result of solar ultraviolet radiation. The surface of the sail will lose electrons and become positively charged. And because the tensile strength of beryllium degrades with temperature, the sail could burst from electrostatic pressure at the earlier 0.05 AU perihelion. Increasing the perihelion distance lowers the electrostatic pressure dramatically and makes the mission feasible.

Can a beryllium sail of this description be launched from the surface, or does it demand space manufacture? We don’t know the answer to that yet, or to the question of whether beryllium is indeed the best material for the sail walls. It seems clear that an inflatable sail will be more massive than other designs despite its advantages in scalability, and it’s also more likely to experience significant damage from micro-meteorites. Plenty of questions remain as we work out the various sail designs and rigging arrangements that may make a fast mission to the Oort Cloud a reality in, the paper suggests, the post-2040 time frame.

The paper is Matloff, Kezerashvili, Maccone and Johnson, “The beryllium hollow-body solar sail: exploration of the Sun’s gravitational focus and the inner Oort Cloud,” now available online.

Comments on this entry are closed.

John HuntNovember 12, 2008, 12:02

six and a half years to get to the Sun’s inner gravitational focus (550 AU)

At this speed, by my calculations, it would take such a craft 3,030 years to reach Alpha Centauri. Clearly, this design wouldn’t work for a science probe since something much faster would be developed in only a fraction of those years. As for a frozen embryo mission, 3,000 years is a stretch for viability of equipment and a great stretch for the viability of frozen embryos or primary gametocyte tissue culture even with protection from cosmic radiation.

For a true interstellar mission it seems as though this could only be a stepping stone if propelled by masers or lasers which would require a tremendous amount of space-based hardware (and hence cost).

Any idea how this sail idea compares with other sail ideas given the same space-based power & beamed propulsion hardware?

Now that is an interesting prospect. While I’m somewhat resigned to the conclusion that a true interstellar mission probably won’t get to its target within my lifetime, I’d settle for detailed images of an extrasolar planet through a gravitational focus telescope…

Any idea how this sail idea compares with other sail ideas given the same space-based power & beamed propulsion hardware?

Beamed propulsion is a different ballgame — the Matloff team is assuming a Sundiver mission to get the needed propulsive kick, and their concept would be nearer term since it requires no laser infrastructure in place. Some of Forward’s laser concepts get up to 10 percent of c, but also make huge demands on our ability to construct the needed engineering throughout the Solar System. Benford-style microwave beaming offers great promise, but I’d consider true interstellar missions using lasers or microwaves as being much further down the road. An earlier Oort explorer seems a reasonable way to shake out many needed technologies.

At 0.004 C, a craft based on the sail technology would have an escape velocity greater than that of the Milky Way, and indeed, greater than that of our local galaxy cluster. A craft as such in the form of a huge world ship or colony ship could roam intergalactic space essentially forever.

However, such a world ship would need a large supply of energy to power its dailly living infrastructure as well as need an extreme degree of insulation to prevent radiative heat loss.

Anyhow, it is nice to see folks actually comming up with real systems backed by numbers that indicate such velocities are possible.

Imagine sending highly shielded pods with frozen human embryos in such systems in trajectories that would spread humanity all over the the volume defined by the radius of the visible universe and beyond.

One can imagine a child being reared by robotic methods after growing in an artificial womb wherein the child would first see the light of day some 100 billion years from now.

This inflatable solar sail technology is one more step that helps the dreams of us spaceheads to eventually become reality.

I wonder if some sort of metalized carbon nanotube based material would work being that carbon nanotubes and similar materials would have a tensile strength up to 600 times that of construction grade steel which has a tensile strength of about 60,000 PSI. This would put the maximum theoretical strength of carbon nanotube or similar materials at 36 million PSI or about 100 times that of Kevlar and Spectra.

Given that metalized Mylar and Nylon toy balloons are usually made of membranous materials as such which are on the order of 10 to 20 microns thick, materials made of top end carbon nanotube material could have the same strength but perhaps be 1/300 as thick. This could result in reflective balloons made of such material that is 33 nanometers thick.

Perhaps the metalic layer could be deposited; by vapor deposition, electrostatically, or by ion beam deposition. It might be possible that more than one element would be deposited on or within the carbon material membrane.

Note that telescope mirrors have included multiple elements or compounds for optimizing their reflectivity. A good example is the use of a layer of magnesium flouride to reflect UV light on a mirror that is also able to reflect visible and IR light. Gold is extremely reflective to a large portion of the IR band. Depositing Gold, Silver, and Magnesium Flouride within the carbon membrane itself may help trap these metals thus effectively increasing their melting points.

The weakest link in any interstellar mission will be the
humans aboard, be they embryos or full grown adults.

I predict most if not all interstellar missions from our
system will be strictly mechanical.

Of course I can see certain human groups wanting to leave
for other systems taking the multigenerational starship
route, but for fast and efficient interstellar exploring, AI
probes will be the way to go.

One of the questions I posed by Email to the team in the mentioned paper was regarding Beryllium:

– Do most materials suffer from this ionization causing electrostatic pressure on the sail at 0.05AU or is this with metals only?

Gregory Matloff wrote:

> Dear Paul
>
> I will let Les deal with most of the questions. But regarding the beryllium sail, detailed analysis has thus far been performed for metalllic sails. Dielectric sails may be better–or they may be worse. Stay tuned!
>
> Regards, Greg

10^7 kg is still not very much mass for a 3000-year voyage. If you assume (as I do) that you’d need at least 50-100 metric tonnes per passenger for structure; food-, water-, and air-processing; power plants; etc., then you’re only talking about a couple of hundred people. On a generation ship for 3000 years? That’s not a lot of genetic diversity in the offspring, and they’d probably be inbred by the time they reached even Alpha Centauri.

It’s an interesting concept, and ideas like this definitely advance the goal of interstellar colonization, but this does not appear to be the final answer.

Frozen embryo missions seem to me to be so much magic pixie dust, because in proposing such ideas, lots of significant problems are handwaved away: getting the frozen embryos safely to the target system is only the first major problem. Not only do you have to provide some kind of artificial environment for the embryos to develop into babies, you then have to provide some way of providing suitable conditions that they grow up into psychologically-healthy adults, which implies the necessity for parents, social interactions, etc. Let’s hope suspended animation for adult humans is a possibility?

The weakest link in any interstellar mission will be the humans aboard, be they embryos…but for fast and efficient interstellar exploring, AI probes will be the way to go.

There are two rationale’s here which need to be separated. For science return, a mechanical probe is the only way to go. It would be much less complex hence easier to design, less likely to fail, and less expensive.

An early “manned” mission serves an entirely different purpose. It has practically nothing to do with exploration. Rather, its purpose is as an insurance policy for the human race in case of an existential catastrophe.

humans aboard, be they embryos or full grown adults…multigenerational starship

It think it helpful to look at these individually rather than lump them together as manned missions. Multigenerational ships are extremely large, complex, expensive and far into the future. Full grown adults are far more massive than embryos, may require life support equipment which can fail, and are lives which can be lost. Frozen embryos are very small, we can already produce children from them, require practically no life support, and risking their “lives” carries much less moral baggage than with adults.

Now we all understand that frozen embryos require complex automated gestation, child rearing, and life support production at destination — admittedly, no small feat. It’s a tough engineering problem but nothing beyond known physics.

I think that an inflatable sail is probably best left for Oort cloud missions. There are probably better ways of reaching Alpha Centauri.

I don’t understand the fuss about solar sails at all. Forget sails. Build a nuclear craft that makes ~0.1c send a few youngsters and a few thousands frozen embryos for genetic diversity. One generation is born en route and they start crank out babies once they arrive. No problems with upbringing by incompetent androids.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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